Gas to Liquids processes that combine a reforming technology for production of synthesis gas with a Fischer-Tropsch process are well known. A variety of reforming technologies and Fischer-Tropsch reactor technologies are available and have differing efficiencies, complexities, scalabilities and costs. Three main technologies for the reforming of synthesis gas are known and they are steam reforming, autothermal reforming and catalytic partial oxidation. For the largest scale processes the reforming technology of choice is usually autothermal reforming as this produces the highest levels of thermal efficiency, operates with the lowest amount of steam and is the most straightforward for building in high capacity single trains for large world scale plants. This is typically combined with a slurry phase Fischer-Tropsch process utilising a cobalt catalyst. The description of the development of these technologies is well documented in texts such as A. P. Steynberg and M. E Dry, Fischer-Tropsch Technology, v 152, Studies in Surface Science and Catalysis, which is incorporated herein by reference.
While the drivers for world scale plants is to achieve competitive pricing through the construction of ever larger plants the high levels of capital investment that are required for such large plants predicates that the plant must be built at large gas reserves capable of producing high rates of gas for many years: fields larger than 1 TcF.
However, much of the world's gas resources are contained within smaller widely separated fields where there is insufficient gas to provide a return on a large scale costly plant. In these circumstances the challenge is to produce a reduced cost plant that is optimised for manufacturing on a small scale with the minimum number of process units.
The concept of a simplified gas to liquids process has been discussed in a series of papers including “A new concept for the production of liquid hydrocarbons from natural gas in remote areas” by K Hedden, A. Jess and T Kuntze, Oil Gas—European Magazine 1994, which is incorporated herein by reference.
The challenge of building large Fischer-Tropsch Reactors is well described in the book by Steynberg and Dry. For the largest scales the difficulties of producing tube sheets with diameters of several meters impacts on the cost and so part of the benefit for slurry bed technology comes from the ease of fabrication of the largest reactors. At the smaller levels of production the reactor complexity can be increased at relatively little incremental cost as the fabrication challenges are lower.
Furthermore where the plants are located in highly remote locations including offshore there are further problems that are hard to solve. For example, while fixed bed Fischer Tropsch reactors provide a good solution with large plants there is a compromise between minimising pressure drop to avoid crushing of the catalyst and excessive compression costs and maintaining a high enough velocity to ensure good heat transfer. The result is a pelleted catalyst of dimensions around 1 mm or more, which suffers a loss of 30% or more of its inherent activity dues to poor catalyst effectiveness attributed to internal mass transfer limitations. The fixed bed reactors must also be tall to ensure high enough velocity through the bed to provide sufficient levels of heat and mass transfer. This presents difficulties in packaging the reactor for transport to the site and issues of gas-liquid distribution if the reactor is moving while located offshore. Egg-shell catalysts have been proposed that locate the cobalt solely in the surface of the catalyst pellet and although this reduces the amount of unused cobalt it is expensive to manufacture and fails to increase the productivity of the reactor.
The challenges of heat transfer, mass transfer and volumetric efficiency for the Fischer-Tropsch reactor design is well described in the paper R. Guettel, T. Turek, Comparison of different reactor types for low temperature Fischer-Tropsch synthesis: A simulation study, Chemical Engineering Science, 64, (2009), 955-964, incorporated herein by reference, which illustrates the advantages and potential of the various technologies that are available for hydrocarbon liquid synthesis. While it is relatively straightforward to produce a cobalt catalyst for Fischer-Tropsch hydrocarbon production that can operate efficiently on the scale of a few grammes of catalyst, this paper highlights the challenges of producing a reactor design capable of maintaining this performance at a commercial scale. Inherently a fixed bed of catalyst cannot operate with high cobalt efficiency unless particles of less than 200 microns are used. However utilising small particles requires using low gas velocities and very short catalyst beds if an excessive pressure drop is to be avoided. This results in poor heat transfer capabilities if the catalyst is simply packed within conventionally sized tubes of 25 mm diameter. The alternative would appear to be to coat the surface of a plate style reactor with particles of catalyst While this solves the problem of the heat transfer and provides more heat transfer surface than is actually needed the construction methods of these types of reactor require that the process gas plus catalyst occupies typically 40% or less of the total reactor volume. Taking into account the manifolding and any pressure containing shell that is require can result in a very low volumetric efficiency of catalyst packing and a high specific reactor capital cost. Some of this loss in efficiency can be recovered through operating the catalyst at higher temperature and with a higher inherent efficiency, but this can result in a reduced catalyst life and lower selectivities to desired hydrocarbon product. It is possible to improve the volumetric loading of the reactor through the use of larger channels within a microchannel device and to then place gas permeable inserts within the channels. For example WO/2004/050799 describes a thin layer of catalyst applied to multi-layer gas permeable structures within micro channels. However thin layers of catalyst, typically 200 micron or less, are still used in order to maintain the catalyst efficiency. There is no sealing provided around the catalyst structures such that the flow of gas is forced (convective) flow through the porous supports. Instead there is only convective flow across the surface of the supports; the gases must diffuse through the thin layer of support and catalyst. While not wishing to be bound by theory it is thought that the lack of forced flow through the porous structure that results in the necessity of only using thin layers of catalyst.
The same restriction is described in other high activity configurations of Fischer-Tropsch catalyst. For example in US 2006/0167120 a high activity catalyst on porous support is described that again proposes a catalyst layer structure where the layer must be 200 microns or less to deliver a high activity catalyst. Without sealing being provided such that there is forced flow through the porous support the system relies solely on diffusion for the gases to reach the catalyst active sites.
Whatever the form of the reactor in the prior art it appears that the restriction on dimensions of the layer exists. Even with slurry reactors that utilise freely moving particles such as in US 2003/0211940 and the catalyst is formed by placement of cobalt on a porous support there still remains a requirement to avoid a thick layer of catalyst if high cobalt activity is to be achieved. This is again because no forced flow through the porous structure is achieved.
The perceived importance of utilizing a thin layer is exemplified in EP 2341120 A1 where by flow of air through the porous support is used in the catalyst manufacturing method to remove excess catalyst and keep the layers within the structure less than 100 microns. Again the reasoning is that it is not possible to utilize a catalyst on a porous support with thick layers.
One alternative proposed that allows a high activity bed to be developed is to use a structured catalyst such as described in Itenberg et al. US2005/0032921/A1, incorporated herein by reference, which utilises a high permeability cylindrical structure with a typical equivalent fixed bed depth of approximately 5 mm. The gas is forced through the porous structure which allows the catalyst to operate without severe mass transfer restrictions. The thermal conductivity of the fused catalyst structure is sufficient to avoid temperatures differentials of more than 5 deg C. building up across the membrane structure.
This goes some way to illustrate the method by which the cobalt structure can be incorporated within the reactor to maintain the cobalt catalyst efficiency but there are several problems with the approach presented.
The solution presented utilises the catalyst material as part of the structural support. It is now well known that even within complex cobalt based catalyst formulations that the catalytic species is simply the metallic cobalt. The presence of other components are there simply to either aid in the production of the optimum size of metallic cobalt crystallite, aid the reducibility of the crystallite produced or to inhibit reaction of the crystallite with the supporting oxide, particularly where aluminum is present. Despite this the correct combination of cobalt, promoters and stabilisers on the supporting oxide is critical in producing an active catalyst. One limitation on slurry phase catalysts is that the formulation must further take into account mechanical strength to produce an attrition resistant catalyst. Similarly the incorporation of the cobalt materials into the main body of the catalyst described by Itenberg et al. is that the catalyst formulation used must be one that can be fused to produce a support structure that is mechanically strong enough to be utilised within a commercial reactor. The problem of producing high mechanical strength catalysts that are capable of surviving either slurry phase attrition or the forces associated with the high pressure drops and packing stresses of a fixed bed process. Additionally sufficient porosity must be maintained in the support structure to accommodate a high concentration of the catalyst material. This further compromises the mechanical integrity of the supporting material.
The requirement to use a thermally conducting catalyst to enable good thermal control of the thick catalyst structure also places limitations on the formulation of the catalyst material, restricting access to the highest activity formulations currently described in the literature.
Additionally the permeability of the catalyst structure is maintained at a very high value to minimise resistance to flow attributed to the velocity of fluid through the pores. This results in a catalyst structure which is susceptible to preferential wetting. Where narrower pores become liquid filled there is a higher resistance to flow which will results in the gas preferentially travelling through the emptier pores reinforcing the effect. If the distance between gas paths exceeds the typically diffusion limiting distance of approximately 0.25 mm then the wetted area of catalyst will show much lower levels of activity due to reduced concentrations of carbon monoxide accessing the catalyst. The diffusion of hydrogen is much more facile than carbon monoxide and so the hydrogen concentration in these wetted areas will rise. The thicker the layer of the catalyst used then the higher the permeability of catalyst layer that is required, and the more susceptible the layer will be to channeling of the gas through the layers, producing a catalyst susceptible to localised loss of activity. This also results in a localised area of high methane production on the catalyst, which is highly undesirable.
Furthermore as the main catalyst body which provides the resistance to flow, that may aid good gas distribution, contains liquid producing cobalt catalyst as a part of normal function, then the loss of flow in any region will increase the local residence time leading to greater production of liquids, and greater resistance to flow. This instability severely limits the use of this technology.
Even once the problems of the mass transfer are resolved and measures are put in place to increase the heat transfer within the catalyst bed it is critical to remove the heat of reaction from the catalyst zone in an efficient manner. How this can be achieved is not described by Itenberg et. al. Intensifying the productivity of a monolith catalyst increased the intensity of the heat transfer required. Consequently it is most beneficial if the increased intensity of the catalyst productivity is accompanied by an increase in the available heat transfer duty. With monolith catalysts this is difficult as the heat transfer is typically provided by re-circulating fluid, with the high pumping costs required or use of multiple adiabatic beds with the associated reactor control problems.
Another alternative is the use of slurry bed technology where the catalyst particle is suspended within liquid product mixture agitated by the gas sparging, which while delivering a reactor that has a higher volumetric loading of cobalt within the reactor and high catalyst effectiveness through the use of small suspended particles suffers from the difficulties associated with catalyst attrition. The fine catalyst particles must be removed from the product solution utilising filtration, either internal or external to the reactor. These filters have a tendency to block as a result of the catalyst attrition inherent to the process. Additionally if the reactor is to be located off-shore where much of the world's stranded gas resources are located and where small scale GTL is an attractive proposition to reduce flaring of gas then movement of the reactor can cause additional problems of liquid and gas distribution.
What is needed is a reactor design that enables a high heat transfer solution to be placed within a Fischer-Tropsch Reactor that enables a high catalyst efficiency to be maintained. It also requires a catalyst support structure that allows the formulations of cobalt catalyst that have high levels of reducibility and activity to be incorporated into the structure without the constraints of mechanical strength and thermal conductivity. Furthermore, to eliminate continuous catalyst replacement a fixed catalyst structure should be utilised. Additionally, achieving a high volumetric concentration of cobalt within the reactor needs to be achieved to produce a high productivity reactor.
Consequently there is a continuing search for a Fischer Tropsch reactor technology, particularly suitable for small scale and off-shore operation that can utilise the latest catalyst formulations in a highly efficient manner. It is an object of at least one aspect of the present invention to obviate or mitigate at least one or more of the aforementioned problems.
It is a further object of at least one aspect of the present invention to provide an improved fixed bed Fischer-Tropsch reactor.
It is a further object of at least one aspect of the present invention to a fixed bed Fischer-Tropsch reactor that incorporates forced flow through a small pore catalyst and high levels of heat transfer that is able to operate with high levels of catalyst effectiveness.